Over the past two decades dielectric recording technology has become firmly established and accepted as a superior means of producing computer generated images on either paper or film based electrographic products. Irrespective of the particular substrate, the product construction is that of a capacitor, as depicted in FIG. 1. The supporting substrate (10) carries an electrically conductive ground plane (12) which is overcoated with a thin dielectric resin layer (14) designed to hold an electrostatic surface charge in an imagewise pattern as depicted in FIG. 1. The supporting substrate is typically paper or plastic. Of the several techniques practiced for coating a ground plane, e.g., sputter deposited semiconductive metal oxides such as indium doped tin oxide, vacuum deposited aluminum, salts of sulfonated low molecular weight polystyrene, or quaternary ammonium salts derived from chloromethylated low D.P. (degree of polymerization) polystyrene, the quaternary salts offer advantages in cost, as well as production simplicity and some advantageous physical properties are obtained also. Unfortunately, in the synthetic process used to manufacture the polymeric quaternary salts, it is inherent that small but objectionable amounts of malodorous trimethylamine may be left in the produce solution. Kirk-Othmer Encyclopedia of Chemical Technology, third edition, John Wiley and Sons, NewYork, N.Y. (1979), Vol. 6, p. 20.
That amine contaminant arises from the final step in the synthesis which is neutralization of excess acid generated as a by-product due to chloromethyl group solvolysis concurrent with the quaternization reaction. Trimethylamine is a principal component of the odor of spoiled fish. Can J. Microbiol., Vol. 20, p. 1754 (1974).
The synthesis of the quaternary conductive resin is set forth below. ##STR1##
Neutralization of the amine hydrochloride and any excess acidity with inorganic bases such as sodium or calcium hydroxide introduces metal salts that can crystallize. These crystals introduce optical and dielectric defects into the dried ground plane coating. Thus, it has been necessary to adopt another means of removing the excess acidity from the conductive resin solution. The technique chosen at the present time appears to be reaction of the hydrogen chloride (in addition to any amine hydrochloride) with an oxirane, specifically ethylene oxide, to form an appreciable quantity of choline chloride (up to 1 wt. %) which has been identified unequivocally via thin layer chromatography as well as GC-MS analysis. Moosnick et al, J. Clin. Invest., Vol. 24, p. 278 (1945).
These neutralization reactions are set forth below: ##STR2##
Despite efforts to remove residual free amine from the resin, its odor can generally be detected over the coated product. These trace quantities of free amine are undesirable because the dielectric imaging products constructed using the conductive resin retain the malodor. Obviously there is a great need for a simple and efficient technique for eliminating the malodor of the recording film.
It is recognized that as a genus, the tertiary amines, e.g., trimethylamine, which are generally used to prepare the quaternized resins have a relatively limited chemistry, i.e., there are few reactions characteristic of that genus. Those reactions typically include oxidation, quaternization, cleavages, and radical abstractions. Of these reactions only quaternization involves conditions considered acceptable for a "clean-up" process for aqueous solutions of quaternary salt polymers.
Already a number of possible means of removing the objectionable trimethylamine odor from electrographic products have been investigated including masking those products with a perfume-like fragrance, attempted purification of the resin via molecular sieve treatment, ion exchange resins, vacuum devolatilization and, precipitation and washing. No approach has been entirely successful.
The difficulty intrinsic in the deodorization is probably best understood by recognizing that the threshold level for nasal detection of trimethylamine is approximately 0.00021 ppm. Karel Vershueren, Handbook of Environmental Data on Organic Chemicals, second edition, Van Nostrand-Reinhold.
In light of the aforegoing explanation and the unsuccessful attempts of the prior art at masking or removing the odor, it now seems likely that the origin of the malodorous amine may be more complex than previously thought since choline salts are described as having an amine odor (Kirk-Othmer, supra) and are known to decompose under various conditions to generate either trimethylamine (Kahane et al, Biochimie de la Choline et de ses Derives, Hermann, Paris (1938)) or N,N-dimethyl ethanolamine. Recognizing this, it has become apparent to the present inventor that the ideal deodorization technique must not only remove trimethylamine but also prevent its slow generation in-situ from a latent source such as choline chloride.